Flexible wiring boards

ABSTRACT

A flexible wiring board are formed by growing metal bumps using a mask film patterned by photolithography. Fine openings can be formed with good precision, therefore, fine metal bumps can be formed with good precision because laser beam is not used to form opening in a polyimide film. After metal bumps have been formed, the mask film is removed and a liquid resin material is applied and dried to form a coating, which is then cured into a resin film. The coating can be etched at surface portions during coating stage to exposed the tops of metal bumps.

This is a Division of application Ser. No. 09/744,572 filed Feb. 28,2001 and issued as U.S. Pat. No. 6,643,923 B1 on Nov. 11, 2003, which inturn is a National Stage of PCT/JP99/04067, filed Jul. 29, 1999. Theentire disclosure of the prior applications is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of flexible wiring boards,particularly to a flexible wiring board capable of forming fine metalbumps and the flexible wiring board manufactured thereby.

2. Description of Related Art

FIG. 4( a)–(d) is a processing diagram showing a process formanufacturing a flexible wiring board of the related art. Referring tothe processing diagram, the process is explained in order. At first, acopper foil is applied on a polyimide film 111 and then the copper foilis patterned into a copper wiring 112 (FIG. 4( a)).

Then, the surface of polyimide film 111 is irradiated with laser beam114 (FIG. 4( b)) to form openings 115 having a predetermined diameter(FIG. 4( c)). At this stage, the top surface of copper wiring 112 isexposed at the bottoms of openings 115, and then copper wiring 112 isplated with copper while the bottom surface is protected with a resinfilm 117 so that copper grows in openings 115 to form metal bumps 116.

When a bare-chip semiconductor device is to be mounted on such aflexible wiring board 110, an anisotropic conductive film is applied onmetal bumps 116 and bonding pads of the semiconductor device are broughtinto contact with metal bumps 116 via the anisotropic conductive filmand pressure is applied. Then, circuits within the semiconductor devicecontact copper wiring 112 via the anisotropic conductive film and metalbumps 116.

Flexible wiring boards of this type 110 are recently much used becausethey are thin and light and freely foldable to provide a high mountingflexibility.

However, residues of polyimide film 111 remain on the top surface ofmetal wiring 112 exposed at the bottoms of openings 115 when openings115 are formed with laser beam 114 as described above. Residues areremoved by immersing the assembly in a chemical solution after openings115 have been formed. However, it becomes more difficult for thechemical solution to penetrate into openings 115 as openings 115 becomefiner, and therefore more difficult to remove residues.

If residues cannot be removed, copper deposition speed varies fromopening 115 to opening 115, whereby homogeneous metal bumps 116 cannotbe formed.

Another problem is variation in the diameter of fine openings 115 (about40 μm to 50 μm) formed by irradiating a rigid polyimide film 111 withlaser beam 114, resulting in variation in the diameter and height ofmetal bumps 116 which causes failure of connection with semiconductorchips.

Still another problem is that it is difficult to reduce the spotdiameter of high power laser beam 114, which makes it impossible to formopenings 115 having a diameter smaller than 40 μm, contrary to therecent demand for finer openings 115.

SUMMARY OF THE INVENTION

An object of the invention is to provide a technique capable of formingfine metal bumps with good precision to overcome the above disadvantagesof the related art.

To attain the above object, the invention provides a process comprisingthe steps of forming a mask film, patterned by exposure and development,on a metal foil and growing metal bumps on the metal foil exposed at thebottoms of openings in the mask film.

In the invention, the step of growing metal bumps is followed by thesteps of removing the mask film, applying a liquid resin material toform a resin material coating on the surface of the metal foil on whichthe metal bumps have been formed, and then curing the resin materialcoating into a resin film.

In the invention, the resin material coating may consist of a pluralityof layered coatings.

When the resin material coating consists of a plurality of layeredcoatings, at least the uppermost coating may be a thermoplastic coating.

In the invention, the surface of the resin material coating on the metalfoil may be located below the height of the metal bumps.

In the invention, the height of said metal bumps from the surface of theresin film may be 35 μm or less.

In the invention, the curing step may be preceded by the step of etchingsurface portions of the resin material coating.

In the invention, the resin material may be a liquid containing apolyimide precursor to form the resin film from a polyimide.

In the invention, the step of forming a resin film may be followed bythe step of partially etching the metal foil from the bottom surface toform a patterned metal wiring.

In this case, a support film may be formed on the bottom surface of themetal wiring.

In the invention, the support film may be partially etched to exposedesired regions of the metal wiring.

Said process may further comprise the steps of bringing bonding lands ofa semiconductor chip into contact with the metal bumps and applying heatand pressure to allow the resin film to develop adhesiveness, wherebythe semiconductor chip is bonded to flexible wiring board.

The invention also provides a flexible wiring board manufactured by theprocess as defined above.

Flexible wiring boards of the invention include those having asemiconductor device connected to the metal bumps.

As defined above, the invention relates to a process for manufacturing aflexible wiring board having metal bumps and the flexible wiring boardmanufactured thereby.

In the invention, an exposable dry film or resist film is applied ordeposited on a metal foil and patterned by exposure and development toform a mask film.

The metal foil is exposed at the bottoms of openings in the mask film,so that metal bumps grow at exposed regions of the metal foil when themetal foil is immersed in a plating solution while its bottom surface isprotected.

The openings in the mask film can be formed in a fine size with highprecision by photolithography. Therefore, the metal bumps can also behomogeneously grown both in width and height.

Then, a liquid resin material is applied and dried or otherwise treatedto form a resin material coating on the surface of the metal foil onwhich the metal bumps have been formed, after which the resin materialcoating is heated or otherwise cured into a resin film, whereby thesurface of the metal foil on which the fine metal bumps have been formedcan be covered with the resin film. If the resin material coating has athickness smaller than the height of metal bumps, the tops of the metalbumps may project from the surface of the resin film withoutpost-treatment.

If the resin material cover the tops of the metal bumps, the resin filmis also formed by curing on the surfaces of the metal bumps, which canbe, however, exposed by polishing or etching.

If etching is used, an uncured resin material coating can be etched toform a resin film with the tops of the metal bumps being exposed.

The resin film may be a thermosetting or thermoplastic film or alaminate of such films as far as it is flexible. From the viewpoint ofdurability or reliability, it is preferable that the resin material is apolyimide precursor to be cured into a polyimide film.

After the resin film has been formed, the bottom surface of the metalfoil can be exposed and etched using a dry film or photoresist as a maskto give a copper wiring. Then, a support film can be formed on thebottom to protect the copper wiring, whereby a flexible wiring boardhaving reliable insulating properties is obtained.

The resin film can be formed to have a multilayer structure by layeringresin material coatings. If the uppermost layer of the resin filmconsists of a thermoplastic resin, the thermoplastic resin film developsadhesiveness upon heating to ensure bonding to a semiconductor device orthe like without using an anisotropic conductive film.

The support film may be formed by applying a sheet-like film or coatinga resin material solution as defined above and curing it. The supportfilm may be patterned to partially expose desired regions of the metalwiring for forming contact regions for connection with another flexiblewiring board or contact regions for wire bonding.

Variation in the height of metal bumps grown by electroplating increaseswith size. Experiments show that the variation is limited to ±3 μm whenthe height above the surface of the resin film is 35 μm or less incontrast to ±5 to ±7 μm observed when said height is 40 μm. When anon-flexible material such as a semiconductor chip is to be connected tometal bumps, the yield is more influenced by variation than bump height.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a)–1(p) are a processing diagram illustrating an example ofprocess of the present invention.

FIG. 2 is a perspective view of a metal bump and a metal wiring.

FIG. 3( a) is a surface microphotograph of metal bumps and theirvicinities on which a resin material coating has been formed.

FIG. 3( b) is a sectional microphotograph of one of the metal bumps.

FIG. 3( c) is a surface microphotograph of metal bumps and theirvicinities on which a resin film has been formed.

FIG. 3( d) is a sectional microphotograph of one of the metal bumps.

FIGS. 4( a)–4(d) are a processing diagram illustrating a process formanufacturing a flexible wiring board of the related art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will now be described with reference to the attacheddrawings.

FIGS. 1( a)–1(p) are a processing diagram illustrating a process of theinvention. The reference 2 in FIG. 1( n) represents an example offlexible wiring board of the invention manufactured by the process, andthe reference 30 in FIG. 1( p) represents the flexible wiring board 2having a semiconductor chip 31 connected thereto.

Referring to FIG. 1( a), a metal foil 11 (a rolled copper foil having athickness of 18 μm here) is initially prepared, and a protective film 12is applied on the bottom surface and a UV-exposable mask film 13 (dryfilm SPG-152 made by Asahi Chemical Industry Co., Ltd.) is applied onthe top surface (at a temperature of 130° C. and a line speed of 2 m/minhere) (FIG. 1( b)).

Then, mask film 13 is exposed to light (exposure light intensity 100 mJ)through a glass mask having a predetermined pattern and developed with achemical solution to form openings 15 at locations corresponding to aplurality of metal bumps 16 described below (FIG. 1( c)). Openings 15can be formed with a precision within ±2.5 μm in diameter and aprecision within ±2 μm in height using a mask having a circular patternof 30 to 50 μm in diameter.

Then, the assembly is immersed in an electrolyte for copper plating andelectric current is applied to grow copper into metal bumps 16 on thetop surface of metal foil 11 exposed at the bottoms of openings 15 (FIG.1( d)). Metal bumps 16 standing on a plurality of openings 15 have ahomogeneous height with good precision because no residues remain on thetop surface of metal foil 11 exposed at the bottoms of openings 15 afterdevelopment. Instead a clean surface is exposed.

Then, mask film 13 and protective film 12 are removed with an alkali(FIG. 1( e)). At this stage, a plurality of mushroom-like metal bumps 16are upright on the top surface of metal foil 11. A carrier film 18 isapplied on the bottom of metal foil 11 (FIG. 1( f)), and then a resinmaterial consisting of a polyimide precursor is applied on the topsurface of metal foil 11 and dried to form a resin material coating 20consisting of the polyimide precursor (FIG. 1( g)).

This resin material coating 20 is convex on metal bumps 16 and theirvicinities, but flat away from metal bumps 16. The thickness of flatregions is smaller than the height of metal bumps 16 so that the tops ofmetal bumps 16 may project from flat regions on resin material coating20.

If resin material coating 20 is too thin with a single application, anadditional resin material consisting of a polyimide precursor may beapplied on the previously formed resin material coating 20 and dried tolayer a second resin material coating thereon. The reference 21 in FIG.1( h) represents such a second resin material coating layered on resinmaterial coating 20. The upper resin material coating 21 here isthermoplastic, contrary to the lower resin material coating 20.

A surface microphotograph of vicinities of metal bumps 16 at this stageis shown in FIG. 3( a). A sectional microphotograph is shown in FIG. 3(b). The tops of metal bumps 16 are covered with resin material coatings20, 21.

Then, an alkaline solution is sprayed on resin material coatings 20, 21to etch the surface. Here, a depth of 2–5 μm from the surface is etchedby spraying at 25° C. for 20 seconds to expose the tops of metal bumps16 (FIG. 1( i)). A plasma cleaner may be used for etching instead ofspraying an alkaline solution.

Then, carrier film 18 on the bottom is removed and then resin materialcoatings 20, 21 are cured by heating (280° C. for 10 minutes) to form aresin film 23 consisting of two polyimide film layers on the top surfaceof metal foil 11 (FIG. 1( j)).

A surface microphotograph of metal bumps 16 at this stage is shown inFIG. 3( c) and a sectional microphotograph is shown in FIG. 3( d). Thesurfaces of the tops of metal bumps 16 are exposed, though indiscerniblefrom FIG. 3( c) and FIG. 3( d). The upper layer of resin film 23 isthermoplastic so that it is not necessary to use an anisotropicconductive film for connecting a semiconductor device or the like.

A photosensitive resin film is applied on the bottom surface of metalfoil 11 and patterned by exposure and development into a predeterminedconfiguration to form a mask film 24 (FIG. 1( k)). Then, the pattern ofmask film 24 is transferred to metal foil 11 by etching, to form a metalwiring 25 (FIG. 1( l)).

This metal wiring 25 has line-shaped wiring regions 25 _(a) andlarge-area contact regions 25 _(b) located at the bottoms of metal bumps16, so that metal bumps 16 can be connected to outer terminals or ICsvia contact regions 25 _(b) and wiring regions 25 _(a).

Mask film 24 is removed (FIG. 1( m)) and a polyimide precursor isapplied on the exposed bottom surface of metal wiring 25 and dried andthen patterned using a photosensitive resist to expose contact regions25 _(b). Then, the assembly is heated and a support film 26 consistingof a polyimide is formed on the bottom of metal wiring 25 to give aflexible wiring board 2 (FIG. 1( n)). The height of metal bumps 16 ofthis flexible wiring board 2 from the surface of resin film 23 is 35 μmor less.

In flexible wiring board 2, the top and bottom surface of metal wiring25 are protected with resin film 23 and support film 26, respectively,and the tops of metal bumps 16 project from the surface of resin film23. The bottoms 27 of contact regions 25 _(b) are exposed.

FIG. 2 is a perspective view of metal wiring 25 and metal bump 16, inwhich polyimide films 23, 26 are not shown.

Next, a process for mounting a semiconductor chip on flexible wiringboard 2, having the structure described above, is explained.

FIG. 1( o) shows the state in which semiconductor chip 31 is ready to bemounted on flexible wiring board 2. A plurality of bonding pads 32consisting of an aluminium thin film are exposed on the surface of thissemiconductor chip 31, and metal bumps 16 formed on flexible wiringboard 2 are provided to face bonding pads 32.

Semiconductor chip 31 is pressed against flexible wiring board 2 viaeach bonding pad 32 of this semiconductor chip 31 in contact with thecounterpart metal bump 16, whereby resin film 23 exposed between metalbumps 16 tightly contact the surface of semiconductor chip 31.

When semiconductor chip 31 or flexible wiring board 2 is heated duringthe pressing step, resin film 23 develops adhesiveness to bondsemiconductor chip 31 to flexible wiring board 2.

When the assembly is cooled as such, semiconductor chip 31 is fixed toflexible wiring board 2 while maintaining electric connection betweenbonding pads 32 and metal bumps 16. The reference 30 in FIG. 1( p)represents a flexible wiring board on which semiconductor chip 31 isfixed.

The tops of metal bumps 16 of another flexible wiring board 2 having asimilar structure may be brought into contact with contact regions 25_(b) of the former flexible wiring board 2, and the flexible wiringboards 2 are connected together by means of adhesiveness of resin film23 of the former flexible wiring board 2.

Table 1 below shows the relation between bump height and connectionfailure when an IC chip (a kind of semiconductor chip) is connected tobumps 16 of flexible wiring board 2 or when flexible wiring boards 2 areconnected together (connection between bumps 16 and bottoms 27 ofcontact regions 25 _(b)). PCT (Pressure Cooker Test) was performed underconditions of 121° C., 2 atm. for 24 hours. All heights of 35 μm or lesspassed PCT without showing any failure point even after PCT.

TABLE 1 Bump height and connection results Comparative ComparativeComparative Description Example 1 Example 2 Example 3 Example 4 example1 example 2 example 3 Bump height (μm)  0 10 10 35 37 40 55 Range ofvariation  1  1  1  2  3  4  6 in bump height (μm) Device bonded WiringWiring IC chip Wiring Wiring IC chip Wiring board board board boardboard Number of success Before PCT 25 25 25 25 25 25 18 points among 25After PCT 25 25 25 25 24 16  0 connection points Connection result PassPass Pass Pass Fail Fail Fail

In flexible wiring board 2 of the invention as described above, a resinfilm is formed after metal bumps 16 are formed, therefore, it is notnecessary to form openings in the resin film with laser beam. Thus, finemetal bumps can be formed with good precision.

Although copper was grown by plating to form metal bumps 16 in the aboveexample, other metals may also be used. Metal foil 11 is not limited tocopper, either. Resin coatings 23, 26 may have a monolayer structure ora two-layer structure and may not be formed from a polyimide.

It is preferable to form a gold coating (thickness of about 1–2 μm) byplating or other means on the surfaces of metal bumps 16 consisting ofcopper. A chip-like semiconductor can be connected to such metal bumps16 via an anisotropic conductive film or the like to prepare a circuitcomponent.

Metal bumps formed on another flexible wiring board can also beconnected to contact regions 25 _(b) to connect flexible wiring boardstogether. Therefore, a plurality of flexible wiring boards of theinvention can be layered.

In the invention, fine metal bumps can be formed with good precision.

A desired shape of opening (for example, square or hexagonal) can beformed because laser beam is not used.

The selection of bump height of 35 μm or less decreases variation inbump height to reduce failure of connection with non-flexiblesemiconductor chips such as IC chips.

1. A flexible wiring board, comprising: a metal foil not patterned; aresin film having flexibility disposed on a top surface of said metalfoil; and metal bumps of which bottoms are connected to the top surfaceof said metal foil, wherein no metal bumps are disposed on a bottomsurface of said metal foil, the bottom surface being flat, and the topsof said metal bumps are projected from said resin film, wherein saidresin film includes a thermosetting resin film and a thermoplastic resinfilm developing adhesiveness upon heating, said thermosetting resin filmcontacts said metal foil, and said thermoplastic resin film is disposedon the surface of said thermosetting resin film, said thermoplasticresin film is exposed, and wherein said thermosetting resin film isdisposed in the vicinities of said metal bumps, sides of said metalbumps are surrounded by said thermosetting resin film, and saidthermosetting resin film is exposed on the surface of the vicinities ofsaid metal bumps.
 2. The flexible wiring board according to claim 1,wherein said thermoplastic resin film and said thermosetting resin filmare polyamide films.
 3. A flexible wiring board, comprising: a metalwiring patterned; a resin film having flexibility disposed on a topsurface of said metal wiring; and metal bumps having correspondingbottoms and tops, the bottoms being connected to the top surface of saidmetal wiring and the tops project through said resin film, wherein saidresin film includes a thermosetting resin film and a thermoplastic resinfilm developing adhesiveness upon heating, said thermosetting resin filmcontacts said metal wiring, said thermoplastic resin film is disposed ona surface of said thermosetting resin film, and said thermoplastic resinfilm is exposed, and wherein said thermosetting resin film is disposedin the vicinities of said metal bumps, sides of said metal bumps aresurrounded by said thermosetting resin film, and said thermosettingresin film is exposed on the surface of the vicinities of said metalbumps.
 4. The flexible wiring board according to claim 3, furthercomprising: a support film disposed on a bottom surface of said metalwiring; and openings formed on said support film, said metal wiringbeing exposed at a bottom of each of said openings.
 5. The flexiblewiring board according to claim 3, said metal bumps are connected tobonding pads of a semiconductor chip, and said semiconductor chip isbonded by means of adhesiveness of said thermoplastic resin film.
 6. Theflexible wiring board comprising first and second flexible wiring boardsaccording to claim 3, being layered, wherein said metal bumps of saidfirst flexible wiring board are connected to said metal wiring of saidsecond flexible wiring board located at the bottoms of openings of saidsecond flexible wiring board, and said first flexible wiring board isbonded to said support film of said second flexible wiring board bymeans of adhesiveness of said thermoplastic resin film.